Chapter 21: Transverse Relaxation of ¹²⁹Xe in the Lung

Hyperpolarized ¹²⁹Xe Magnetic Resonance Imaging (MRI) is a promising approach for visualizing the lungs non-invasively, particularly to follow longitudinal changes in lung function and anatomy without the accumulated radiation dose associated with X-rays. Hyperpolarization is typically achieved using spin-exchange optical pumping techniques resulting in approximately a 10 000-fold improvement in available magnetization compared to conventional Boltzmann polarizations. This substantial increase in polarization allows high spatial resolution images of the lung to be obtained with excellent temporal resolution. Since the available magnetization is non-renewable, careful attention to the MRI method is required to achieve best image quality and extract useful information. In particular, the time constants governing the relaxation of the transverse magnetization (T₂* and T₂), are important to consider. This chapter provides an overview of the principal mechanisms of ¹²⁹Xe gas transverse relaxation in the lung, including how gas transverse relaxation times affect lung MRI image quality. The choice of optimal field strength is discussed and the role of low static magnetic fields is highlighted. The application of low field ¹²⁹Xe gas transverse relaxation times to estimation of apparent diffusion coefficients (ADC) and alveolar partial pressure of oxygen (p_AO₂) is described and the effects of transverse relaxation of ¹²⁹Xe occurring in the dissolved phases of the lung (i.e. tissue and blood compartments) are discussed.